Fish of the Forest: Large Wood Benefits Salmon Recovery

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When spring arrives in the Pacific Northwest, bright tender leaves unfurl, snowdrops and balsam root push up through winter’s brown blanket, and neighbors spill from their homes after holing up for the dark, damp months of the year. We awaken, stretch, and emerge; eager to reconnect after a winters-long hermitage.

In cold waters running from mountains to sea, another emergence is happening. From March through June, tiny salmon fry rise from gravel nests, their stomachs still distended yolk sacs. As they draw down their yolk stores, juvenile salmon begin feeding on the stream’s insect life. There are six anadromous salmon species in the Pacific Northwest — Chinook, coho, sockeye, pink, chum, and steelhead. As juveniles, each species specializes in a slightly different cuisine and method of foraging food, as well as the amount of time they spend in freshwater before heading out to sea.

In turn, each foraging strategy provides a different evolutionary advantage, as well as a different suite of vulnerabilities. The evolutionary trade-off for juvenile salmon essentially boils down to two choices: spend years growing as large as you can in food-limited freshwaters to avoid being eaten upon ocean arrival, or head directly to the coast where plentiful food encourages rapid growth, but hungry predators abound.

Historically, the advantages and vulnerabilities of the two strategies were well balanced. But today, human demands on the landscape have shifted the balance. A typical Chinook or coho strategy, for example, is to spend extensive time rearing in quiet freshwater streams before migrating to the ocean. Activities that impact small streams and rivers, such as logging, development, pollutants and climate change, therefore directly threaten these species. As a result, these fish appear on the US federal endangered species list in Washington, Oregon, California, and Idaho. The freshwater life strategy that once promoted a size advantage upon ocean entry, is now a significant liability.

Ecological Portfolios: Maintaining Stability in the Face of an Uncertain Future

Similar to efficient financial portfolios, biological systems depend upon a diversity of ecosystem components (i.e. marshes, streams, forests, rivers) to guard against volatility. The idea is that a wide assortment of diversifying features in biological systems stabilizes their performance, just as diversification across assets can stabilize returns to stock portfolios. Simply stated, you wouldn’t put all your financial eggs into one short-sale basket. Financial security and growth requires a far more complex portfolio.

Beneficial “portfolio effects” play out in virtually every ecosystem, but are especially well documented for salmon and the landscapes they depend on. By studying pristine landscapes, such as Alaska’s Bristol Bay region, it is now clear to scientists that the best ecological tool for managing risk and recovering salmon is to maintain a thick portfolio of complex habitats across the landscape.

For Pacific Northwest salmon, the portfolio of available ecosystems is dangerously thin, unable to provide diversified insurance against ever encroaching human impacts and an uncertain future.

To enhance natural river processes critical to salmon and watershed recovery, TNC and its partners are reintroducing fallen trees to streams and floodplains throughout coastal Washington, Puget Sound, and the Central Cascades. This restoration technique ranks as one of the most urgent actions needed for the recovery and future resilience of salmon because it promotes a complex portfolio of aquatic habitats. Once in the water, large wood initiates log jams that in turn increase natural scour, create new pools and deepen existing ones, provide slow-water refuge for juvenile fish, create gravel beds for nesting, trap nutrients in streams, and increase food availability.

Why Do River Systems Need More Wood? A History Lesson on Stream Cleaning and River Simplification

Consider your mental image of a scenic river tumbling from glaciated mountain headwaters, rushing through forested foothills, and meandering across floodplains before meeting the sea. Does your image include downed trees and piled up stumps, or log jams spanning entire river channels? Does it include a free-flowing river eating through forest as it traverses across lowland valleys? It not, you are in good, but perhaps flawed company.

Most of us have never experienced an “uncleaned,” freely meandering river.

As a global society, we have spent the last 100 years intentionally removing wood from stream and river channels in order to improve navigation, water conveyance, and transportation of goods. Wood removal was once even thought to promote fish passage. As a result, the amount of large wood in streams is far below natural levels. Historically, coastal river travel required navigating — on average — one downed tree every 6 feet. Today, you’d be lucky to find one tree every quarter mile.

The cumulative effect of large wood removal projects over the past century has been to severely simplify river systems. If complexity promotes ecological resilience, then river simplification can only hinder it.

Rebuilding Resilience in Riverine Landscapes: The Scientific Case for Large Woody Debris

With rivers confined between levee walls and riparian corridors stripped of large trees, it could be a half century before restoration of riparian and river processes naturally contribute wood to river systems again. In heavily damaged landscapes like coastal Washington, rebuilding river complexity and resilience requires a bit of a jumpstart.

Large wood plays a critical role in maintaining physical complexity in stream, river, and floodplain ecosystems. In the rush of water tumbling downstream, large wood accumulations create critical areas of deep, quiet water. These pools become repositories for fine sediments and gravel, influencing the amount of spawning grounds available for salmon and maintaining streambed elevations. Without quiet water, streams scour into deeply incised channels that are disconnected from floodplains, gravel deposits and side channels. Heavily scoured stream systems are apparent in areas damaged by destructive logging practices, particularly splash dam operations that were once used to transport timber out of the backcountry. On TNC’s Hurst Creek Preserve on Washington’s Olympic Peninsula, splash dam scouring and wood removal dropped the bottom of the river by eight feet — right down to the bedrock.

Pools created by large wood also promote the deposition of decaying leaves, sticks, and other particles. In this way, streams hold onto nutrients that would otherwise be flushed downstream. Scoop your hand through the debris at the bottom of a pool, and you’ll find a teeming community of tiny worms and insect larvae. Meanwhile, the wood itself becomes a perfect surface for growing algae — the slippery brown scum that makes river walking treacherous. While perhaps an unappetizing menu, decaying leaves and brown scum represent the base of the food web upon which juvenile salmon depend.

In floodplain valleys and estuarine deltas, large log jams are island builders and river dividers. Log jams at the head of gravel bars eventually become large, forested islands. These islands force the river to meander across floodplain valleys, cutting through riparian forest in a braid of channels that reinforces the cycle of erosion, treefall, and island formation. The cycle ensures a complex mosaic of gravel bars, islands, floodplains, swift river chutes, quiet pools, and backwater streams — a kaleidoscope of habitats that offer salmon refuge from predators, feeding and spawning grounds, and safe routes to the sea. The entire process hinges on constant movement and constant change, creating an ecological portfolio that rivals the best the stock market has to offer.

New Shelter: Building a Place for Salmon to Grow

Imagine running on a treadmill while trying to grab your lunch off a moving conveyor belt gliding along in the opposite direction. Now, imagine that the very little food on that conveyor belt is all mixed up with things that look like food, but isn’t actually edible. Also, you’re only allowed to open your eyes every 5 seconds to see what’s coming down the pipe, while your competitive cousin aggressively knocks away your arm as you reach for food (that’s a coho trait, by the way). Finally, you are ravenous because you are training for an ultramarathon.

While perhaps a great exercise in agility and patience, this is a terrible way to calorie load and build body mass. Unfortunately, this scenario emulates the challenge of feeding and growing in simplified river systems for juvenile salmon.

A river system without obstructions flows smoothly and evenly, perfect for a leisurely canoe paddle. The problem, however, is that drift-feeding fish must constantly expend swimming energy while they forage for insects drifting downstream.

Fish growth, it turns out, relies on a complexity of streamflow velocities because fish use fast and slow moving water in different ways. Fast water is the primary food delivery system, shuttling aquatic caddisfly larvae, diptera, terrestrial insects, and sometimes even shrews down nature’s conveyor belt. In contrast, quiet waters and stream edges afford fish with critical resting habitat and refuge from predators. However, resting grounds are not viable locations for finding enough food. Fish must balance their need for food with their need to rest, otherwise growth is stymied.

Log jams, boulders, and enormous tree stumps break up the flow of a river, creating swift chutes, back eddies, and deep pools in close proximity to one another. Here, a fish can belly up to the border between low speed water and swift currents, darting out to snatch insect prey drifting downstream, then retreating to rest and digest. For a fish trying to grow, this is far more efficient than constantly swimming upstream against the current. It allows fish to optimize the bioenergetics of their environment — balancing energy intake with energy used.

While the concept of resting habitat has long been associated with large wood in streams and rivers, scientists have only recently integrated stream hydrodynamics and bioenergetics models to assess the influence of habitat complexity generated by large woody debris. It turns out that most river stretches are too swift for fish to grow because they expend the majority of their energy simply trying to hold their place in the river. The results seem surprising at first for an aquatic species, until you realize that you cannot grow zucchini in deep shade, cacti in a marsh, or chickens on tree bark. Each species has evolved to meet its own environmental optimum.

Scientists don’t yet know the full answer to this question, but emerging results are encouraging. For starters, Chinook grow just as well in streams with one third less wood than natural conditions. Even better news is that large woody debris doesn’t need to slow streams down very much in order to have far reaching impacts. On the Merced River, the reintroduction of large wood slowed average stream speed by only 5-20%. This small reduction in speed, however, effectively quadruples the potential area of the river where salmon can actually grow. That’s the equivalent of adding 8 extra miles to a 2-mile long stream.

Setting the Stage for Recovery: Celebrating Humans and Nature

With the ambitious goal of tripling salmon returns in key river basins by 2020, these results are especially gratifying for TNC Washington, which has large wood projects throughout coastal Washington, Puget Sound, and the Central Cascades.

The projects began on TNC’s Ellsworth Creek Preserve, which lies in the heart of timber country. The nearly 8,000-acre preserve encompasses an ancient stand of old-growth forest, as well as acres of heavily logged second growth. Adjacent to the Willapa National Wildlife Refuge and a rare un-diked estuary on the Naselle River, the landscape brims with salmon potential.

When TNC purchased the property, investment companies were sweeping into the timber market. Purchasing land just long enough to turn a profit, these companies left struggling logging communities and over-worked landscapes in their wake.

It is here that innovative minds found a way to interweave the long-term needs of humans and nature. To promote watershed restoration, TNC has removed roads that once ran along the banks of Ellsworth, torn out stream barriers to fish passage, and added 49 meticulously engineered log jams to promote habitat complexity and recover hydrologic processes along the creek.

Make no mistake. This is invasive, challenging work that requires enormous skill, laser focus, and devoted partnerships. The process requires heavy equipment in streambeds, chainsaws felling riparian trees, and cabled zip-lines plunging tangles of logs into stream channels. The operation appears the antithesis of the wilderness leave-no-trace ethic. But despite the heavy machinery needed to fell trees and place jams, nowhere will you see the concrete blocks, bulkheads, and steel anchor cables of so many restoration projects. This is an organic operation, meant to be self-sustaining once the restoration crews pull back their equipment.

If you are thinking about the high cost of that work, you’ll be especially interested to hear what helps to fund it. As part of its forest restoration objectives, TNC’s landscape prescription includes thinning second growth tree farms. The pairing provides much needed economic stability for skilled natural resource workers, a source of revenue for TNC, and accelerates ecological resilience for endangered salmon, aquatic ecosystems and coastal rainforest.

TNC is now advancing the watershed restoration model it developed in Ellsworth Creek to two other areas — the Hurst Creek Preserve and the Manastash-Taneum Creek Preserve at the headwaters of the Yakima River on the eastern slopes of the Cascades. Although only time will tell the extent of TNC’s success, ambitious large wood installation projects are certainly setting the stage for river and salmon recovery.

Emily Howe, PhD, is the aquatic ecologist for the Washington chapter of The Nature Conservancy.

Join the Discussion

Thank you for eloquently explaining the big wood concept. I am curious if you have had a chance to review the improvements underway for our small Arboretum Creek in Seattle? I am trying to find out if there is anything additional that can be done to improve the habitat for salmon. I realize we need more, clean, cool water. Still i think improvements for salmon in city streams is critical to creating a society that is in-tune with nature. Larry Hubbell http://www.unionbaywatch.blogspot.com

“There are six anadromous salmon species in the Pacific Northwest — Chinook, coho, sockeye, pink, chum, and steelhead” … if you count steelhead, you logically should count anadromous cutthroat, and might mention anadromous salvelinus species